Feingold is currently serving as the U.S. special envoy for the Great Lakes and the Democratic Republic of the Congo, but many Democrat expect him to return to run. As Nathan Gonzalez pointed out , Feingold would have to figure out how to overcome the internal campaign problems that scotched his re-election last time.

If he does return, Democrats will likely give Feingold a clear shot at Johnson. Rep. Gwen Moore, D-Wis., told CQ Roll Call earlier this month, “no one would primary him.”

But if Feingold stays out, the race could look very different. Moore implied she might consider a run. Democrats also discuss Rep. Ron Kind, D-Wis., as a possible contender.

On the Republican side, Johnson is among the most vulnerable Republican incumbents this cycle. He has made no secret of his distaste and frustration with the workings of the Senate.

What’s more, Johnson has $670,000 in his campaign account — not a huge amount for a vulnerable senator facing competitive race. He also said last month he did not intend to support his campaign with his own wealth like in 2010, when he spent almost $9 million of his own money on the race.